Applying quote fails with Pillow>=4.2.0

[Elon Bing]

Starting with Pillow 4.2.0, Variety fails when trying to apply a quote:

ERROR: 2017-07-07 23:38:08,246: apply_quote() 'Could not apply quote:'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/variety/VarietyWindow.py", line 1259, in apply_quote
    QuoteWriter.write_quote(self.quote["quote"], self.quote.get("author", None), to_set, quote_outfile, self.options)
  File "/usr/lib/python2.7/site-packages/variety/QuoteWriter.py", line 46, in write_quote
    raise exception[0]
IOError: cannot write mode RGBA as JPEG

This seems to be caused by the following change:

"Before Pillow 4.2.0, attempting to save an RGBA image as JPEG would discard the alpha channel. From Pillow 3.4.0, a deprecation warning was shown. From Pillow 4.2.0, the deprecation warning is removed and an :py:exc:`IOError` is raised." (https://github.com/python-pillow/Pillow/blob/master/docs/releasenotes/4.2.0.rst#removed-deprecated-items)

Explicitly converting the image-with-quote to RGB before saving seems to solve the issue (patch attached)

[James Lu (jlu5)]

Looks good, and there are no apparent side effects on older (4.x) versions of Pillow. Merged in https://bazaar.launchpad.net/~variety/variety/trunk/revision/595, thanks!